Recent automobiles, etc., load electronic components for control, and carry out high-speed electronic control with a high accuracy.
For example, automatic vehicles carry out electronic control applying hydraulic pressure, to realize automatic transmission.
Automatic vehicles load a solenoid for electronically controlling hydraulic pressure, and a solenoid drive unit for passing a current through the solenoid.
Art concerning the solenoid drive unit is recited in for example, FIG. 2 of Unexamined Japanese Patent Application KOKAI Publication No. H8-240277. The content of this publication is incorporated herein.
FIG. 8 is a diagram showing an outline structure of a conventional solenoid drive unit shown in FIG. 2 of the aforementioned publication.
A solenoid drive unit 80 is constituted by an N-channel type MOS transistor (hereinafter referred to as NMOS) 81, a battery 82, a control circuit 83, a diode 84, a solenoid 85, a resistance 86, an amplifier 87, a resistance 88, and a capacitor 89. The solenoid drive unit 80 passes a current to a coil of the solenoid 85, and drives the solenoid 85, by exciting the current.
A drain of the NMOS 81 is connected to a positive electrode of the battery 82. A gate of the NMOS 81 is connected to the control circuit 83. A source of the NMOS 81 is connected to a cathode of the diode 84 and one end 85a of the solenoid 85.
A negative electrode of the battery 82 is connected to a ground. The control circuit 83 provides a control signal S83 to the gate of the NMOS 81. An anode of the diode 84 is connected to the ground. Another end 85b of the solenoid 85 is connected to one end of the resistance 86.
The other end of the resistance 86 is connected to the ground. The resistance 86 is a conversion circuit that converts a current that passes through the solenoid 85 to a corresponding voltage. Both ends of the resistance 86 are connected to a positive input terminal (+) and a negative input terminal (−) of the amplifier 87.
An output terminal of the amplifier 87 is connected to one end of the resistance 88. The other end of the resistance 88 is connected to one electrode of the capacitor 89. The other electrode of the capacitor 89 is connected to the ground.
FIGS. 9A to D are wave form diagrams for describing the operation of the solenoid drive unit 80 in FIG. 8. Operation of the solenoid drive unit 80 will be described with reference to FIG. 9.
In the solenoid drive unit 80, as shown in FIG. 9A, the control circuit 83 provides a control signal S83 that repeats a high level (hereinafter referred to as “H”) and a low level (hereinafter referred to as “L”) to the gate of the NMOS 81.
When the control signal S83 is “H”, the NMOS 81 is turned on, and connects one end 85a of the solenoid to the positive electrode of the battery 82. By this, a power source current I1 passes through the negative electrode of the battery 82 via the positive electrode of the battery 82, the resistance 86, and the ground.
When the control signal S83 changes to “L”, the NMOS 81 is turned off. When the NMOS is turned off, a counter electromotive force occurs at the solenoid 85. By this counter electromotive force, a regenerative current I0 passes through the anode of the diode 84, the cathode of the diode 84, the solenoid 85, the resistance 86 and a loop of the ground, from the ground.
By the current I1 and the regenerative current I0 passing, a voltage proportional to the current that passes through the solenoid 85, is generated at both ends of the resistance 86.
The amplifier 87 amplifies the difference of voltage of the voltage input to the negative input terminal and the voltage input to the positive input terminal, and outputs a voltage signal 87 that pulsates, as shown in FIG. 9C.
The smoothing circuit that is constituted by the resistance 88 and the capacitor 89, smoothes and outputs, as shown in FIG. 9D, the voltage signal S87 that the amplifier 87 outputs. The smoothed voltage signal S87 output from the smoothing circuit is for example, fed back to the control circuit 83.
The control circuit 83 changes the duty ratio of the control signal S83, based on the smoothed voltage signal fed back from the smoothing circuit. Namely, the control circuit 83 carries out PWM (Pulse Width Modulation) control. By this, the current that passes through the solenoid 85 is optimized.
The output of the smoothing circuit is for example, A/D converted, and provided to a processor for vehicle control, which is not shown in the drawings.
The conventional solenoid drive unit 80 has the problems of below. The NMOS 81, witch is a switching element, and the resistance 86, which is a conversion circuit, both generate heat, because a current for driving solenoid 85 passes through. Therefore, the NMOS 81 and the resistance 86 are embedded in an Electronic Control Unit (ECU) as different components. However, by separating the two components, the temperature in the ECU becomes uneven, and because temperature of each component varies, it is difficult to detect a current (a current that passes through the solenoid 85) accurately.
To accurately detect a current, a component for temperature correction, which is not shown in the drawings, is placed in the ECU. Therefore, the number of components increases, and low-cost of the ECU is difficult. Because the solenoid drive unit is constituted by a plurality of components, miniaturization of the ECU is also difficult.
The same problems exist not only in the solenoid drive unit, but also in other drive units of actuators, such as a motor, etc.